Importance of monodentate mono-ligand designs in developing N-stabilized Pd catalysts for efficient ambient temperature C[sbnd]C coupling: Donor strengths and steric features

Article abstract of DOI:10.1016/j.mcat.2019.110398

Unfriendly temperature profiles and costs of carbon coupling catalysis, which pose challenge to both synthetic organic chemists as well as industrial applicability, motivated our design of new monodentate N-donors as support ligands for the purpose of constructing ambient temperature precatalysts that are molecularly close to the hypothetical active forms. Therefore, a series of sterically varied monodentate N-donor imidazoles (1–7) and oxazoles (8–9) have been synthesized and their N-donor strengths, which were estimated as pK_as, are systematically varied from 0.9 to 8.5 by substituent variations. Eleven target mono-ligand complexes (1-PdCl₂MeCN – 9-PdCl₂MeCN, 6-PdCl₂PhCN and 7-PdCl₂PhCN) and six trans-bis-ligand complexes (1₂-PdCl₂, 2₂-PdCl₂, 3₂-PdCl₂, 6₂-PdCl₂, 1₂-Pd(OAc)₂ and 2₂-Pd(OAc)₂) were isolated and catalytically studied along with PdI₂(PPh₃)₂. Results of coupling reactions, which were conducted both via in situ ‘Pd(II) salt + ligand’ approach and by use of the precatalysts, show that the mono-ligand precatalyst designs (1-PdCl₂MeCN – 9-PdCl₂MeCN, 6-PdCl₂PhCN and 7-PdCl₂PhCN) represent a true catalyst improvement initiative among the phosphine-free catalyst community; i.e. yields approaching 100% (TOF ≈ 2000) at 0.2 mol % catalyst loading, 45 °C and within 15 min. On the other hand, the complexes with trans-bis-ligand coordination were inactive at ambient temperatures. Therefore, it was concluded that coordinative saturation, which results from implementing two or more ligand equivalents or use of polydentate ligands on palladium, should be strongly discouraged. Such saturation necessitates the undesirable and avoidable high temperature necessities, long reflux durations and needlessly high catalyst loadings. Correlation between catalyst activity and donor strengths or steric properties were analysed leading to important conclusions. The catalyst design also supported coupling of activated aryl chlorides from 60 °C while Heck coupling activities were observed only at the early minutes of reactions.

Full text of DOI:10.1016/j.mcat.2019.110398

MolecularCatalysis473(2019)110398
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Importance of monodentate mono-ligand designs in developing N-stabilized
Pd catalysts for efficient ambient temperature CeC coupling: Donor
strengths and steric features
⁎
⁎⁎
Abiodun Omokehinde Eseolaa,b, , Helmar Görlsa, Winfried Plassa,
a Institut für Anorganische und Analytische Chemie, Friedrich-Schiller-Universität Jena, Humboldtstr. 8, D-07743, Jena, Germany
b Materials Chemistry group, Department of Chemical Sciences, Redeemer’s University Ede, Osun State, Nigeria
A R T I C L E I N F O
A B S T R A C T
Keywords:
Unfriendly temperature profiles and costs of carbon coupling catalysis, which pose challenge to both synthetic
organic chemists as well as industrial applicability, motivated our design of new monodentate N-donors as
support ligands for the purpose of constructing ambient temperature precatalysts that are molecularly close to
the hypothetical active forms. Therefore, a series of sterically varied monodentate N-donor imidazoles (1–7) and
oxazoles (8–9) have been synthesized and their N-donor strengths, which were estimated as pKas, are system-
atically varied from 0.9 to 8.5 by substituent variations. Eleven target mono-ligand complexes (1-PdCl2MeCN –
9-PdCl2MeCN, 6-PdCl2PhCN and 7-PdCl2PhCN) and six trans-bis-ligand complexes (12-PdCl2, 22-PdCl2, 32-
PdCl2, 62-PdCl2, 12-Pd(OAc)2 and 22-Pd(OAc)2) were isolated and catalytically studied along with PdI2(PPh3)2.
Results of coupling reactions, which were conducted both via in situ ‘Pd(II) salt + ligand’ approach and by use of
the precatalysts, show that the mono-ligand precatalyst designs (1-PdCl2MeCN – 9-PdCl2MeCN, 6-PdCl2PhCN
and 7-PdCl2PhCN) represent a true catalyst improvement initiative among the phosphine-free catalyst com-
munity; i.e. yields approaching 100% (TOF ≈ 2000) at 0.2 mol % catalyst loading, 45 °C and within 15 min. On
the other hand, the complexes with trans-bis-ligand coordination were inactive at ambient temperatures.
Therefore, it was concluded that coordinative saturation, which results from implementing two or more ligand
equivalents or use of polydentate ligands on palladium, should be strongly discouraged. Such saturation ne-
cessitates the undesirable and avoidable high temperature necessities, long reflux durations and needlessly high
catalyst loadings. Correlation between catalyst activity and donor strengths or steric properties were analysed
leading to important conclusions. The catalyst design also supported coupling of activated aryl chlorides from
60 °C while Heck coupling activities were observed only at the early minutes of reactions.
N-Donor
Low temperature reaction
Palladium catalyst design
Carbon coupling
Ligand effects
Introduction
or even presence of coordinative heteroatoms on catalysis substrates
[22–24] have been established, which is also proven by commerciali-
Palladium-mediated coupling reactions have provided invaluable
routes to otherwise impossible synthetic organic chemistry procedures
[1–6]. Contributions have continued to appear in the areas of catalyst
development [7,8], fine-tuning of successful catalyst systems [9], me-
chanistic studies [10,11], exploration of new substrate types [4,12],
making the process greener [13] and catalyst reusability [14,15] among
other research interests. Of all the commonly utilized CeC coupling
procedures, Suzuki and Heck coupling methods enjoy higher patronage
owing to simplicity of reaction conditions and widespread availability
of the required substrates [16–18].
zation of ligand-supported palladium precatalysts like the phos-Pd and
PEPPSI™ series (Scheme 1) [25,26]. However, in addition to high costs
caused by sophisticated ligand architectures [27–29], these commer-
cialized phosphine- and / or organometallic-based precatalysts are also
concerned with air and moisture sensitivity problems. Furthermore,
high temperatures and / or long reflux durations [30,31], which con-
stitute further energy drawbacks for industrial deployment, are still
often necessary for many of these precatalysts [32–34]. Quoting Farina
on what the industry would consider as a practical catalyst, he said “If
an approximate cost scenario for the final target molecule is known, it is
much easier to set a target TON (i.e. Turnover Number)…’’ [35].
The supportive influence of coordinated ligand backbone [19–21]
⁎ Corresponding author at: Institut für Anorganische und Analytische Chemie, Friedrich-Schiller-Universität Jena, Humboldtstr. 8, D-07743, Jena, Germany.
⁎⁎ Corresponding author.
E-mail addresses: biodun.eseola@uni-jena.de, bioduneseola@run.edu.ng (A.O. Eseola), sekr.plass@uni-jena.de (W. Plass).
https://doi.org/10.1016/j.mcat.2019.110398
Received 28 March 2019; Received in revised form 6 May 2019; Accepted 8 May 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.

A.O. Eseola, et al.
MolecularCatalysis473(2019)110398
Scheme 1. Some commercialized chloridopalladium(II) and or phosphine-based precatalysts for carbon coupling catalysis.
Therefore, motivation exists for study and development of palladium
catalysts that support ambient condition performances while also being
simple, affordable and yet efficient. Therefore, this submission aims to
explore complexes of stable, cost effective and easily synthesizable N-
donor ligands.
Donors such as acetonitrile (pKa = –11) [46] and benzonitrile
(pKa = −10) [47] are often employed for preparing starting materials
of complexation reactions because they are weakly basic and easily
displaceable [48]. Consequently, they are considered as complementary
co-ligands in this study. Inspired by our previous experiences on the
electronic and structural diversity of azole ligands [49–53], we present
herein the results of our ambient temperature studies of a series of
monodentate azole N-donor ligands 1–9 (Scheme 2) as organic back-
bones towards palladium catalysed Suzuki and Heck coupling catalysis.
Dependence on systematic variation of electronic, steric and co-
ordinative saturation is analysed.
A careful examination of the coordination spheres of the commer-
cialized phosphine- or organometallic-based palladium precatalysts
shows that there is always only one strong donor (e.g. phosphine,
pKa ≥ 7) while the remaining coordination donors often consists of
easily detachable donors such as halide, carbon and amine donors
(Scheme 1). Therefore, it is conceivable that their high performance is
associated with generation of monodentate mono-ligand active species
during catalysis [36]. Furthermore, the few existing reports involving
monodentate N-donor ligands in palladium coupling catalysis have
been isolated as bis-ligand complexes, which are only active under high
temperature profiles (Table 1, A–F) [37–41]. Even our previously re-
ported dichloridopalladium(II) complexes of 2-(1H-imidazol-2-yl)pyr-
idine and 2-(1H-imidazol-2-yl)phenol chelates (Table 1, G–H) also gave
encouraging time, temperature and yield parameters relative to the
precatalysts A–F [42–44]. Doubtful low temperature catalysis data for
chelate or cyclometallated are also being reported [45]. Therefore, in
order to overcome thermal barriers during transformation from pre-
catalyst to active forms, it is imaginable to design precatalysts with
molecular compositions that is already close to the anticipated active
forms.
Experimental
General information
All starting materials for syntheses as well as substrates for catalytic
experiments were obtained commercially as reagent grade and used as
supplied. Elemental analyses were performed on Leco CHNS-932 or El
Vario III elemental analysers. 1H and 13C NMR spectra were recorded
on Bruker ARX 300–600 MHz instrument using deuterated solvents. IR
spectra were recorded on Bruker Equinox spectrometer equipped with a
diamond ATR unit. Measurement of UV absorption as well as the
fluorescence excitation and emission spectra were recorded on Varian
Carry 5000 UV–vis-NIR spectrophotometer and Jasco FP 6300
2

A.O. Eseola, et al.
MolecularCatalysis473(2019)110398
Table 1
Complexes A – H of previously reported N-donor palladium precatalysts and their catalytic behaviours for aryl bromides [37–43,54–56].
Precatalyst
A
B
C
D
Yield
68 %
99 %
47 %
57 %
Time
16 h
1 h
4 h
1 h
Temp.
90 °C
100 °C
0.1 mol %
140 °C
0.1 mol %
100 °C
0.1 mol %
Pd loading
Product
0.1 mol %
Precatalyst
E
F
G
H
Yield
78 %
86 %
78 %
95 %
Time
12 h
4 h
0.5 h
0.5 h
Temp.
120 °C
0.1 mol %
100 °C
2.0 mol %
100 °C
0.2 mol %
85 °C
Pd loading
Product
0.2 mol %
spectrofluorometer respectively. Measurements were typically obtained
from 10−5 M solution of the ligands in a 1 cm × 1 cm quartz cuvette
equipped with a tiny magnetic bar.
IR peaks (ATR, cm−1): ν 2970s (CeH alkyl), 2933s (CeH alkyl), 1615s
(C]C aromatic), 1477vs, 1411vs, 849vs, 608 s. 1H NMR (400 MHz,
(CD3)2SO) δ 11.33 (s, 1 H), 6.90 (s, 1 H), 2.47 (t, J = 7.5, 4H), 2.26 (s,
3H), 2.04 (s, 6H), 1.13 (t, J =7.5 Hz, 6 H). 13C NMR (101 MHz,
(CD3)2SO) δ 142.36, 137.91, 137.55, 130.44, 128.17, 21.16, 20.34,
15.52. MS (EI) m/z 242 (M+, 100%): 227, 211, 146.
Syntheses of ligands
2-(Anthracen-9-yl)-4,5-diethyl-1H-imidazole (2): Anthracene-9-
Since the ligands were obtained according to similar procedures,
only the synthetic procedure for ligand 1 is described while the ana-
lytical data are presented for all ligands.
carbaldehyde
(2.00 g,
9.7 mmol),
hexane-3,4-dione
(5.9 mL,
48.6 mmol) and ammonium acetate. Chromatography purification by
silica gel and (i) 100% CHCl3 and (ii) 1:2 tetrahydrofuran / n-hexane.
(7.50 g, 97.0 mmol) (Yield =0.9 g, 31%). Mp. 153 °C. Selected IR peaks
(ATR, cm−1): ν 3052 s (CeH aromatic), 2971s (CeH alkyl), 1603s
(C = C aromatic), 1522s, 1443vs, 733vs. 1H NMR (400 MHz, (CD3)2SO)
δ 12.03 (s, 1 H), 8.71 (s, 1 H), 8.15 (d, J =8.1 Hz, 2 H), 7.81 (d, J
=8.4 Hz, 2 H), 7.58 – 7.46 (m, 4 H), 2.64 (q, J =7.5 Hz, 4 H), 1.25 (t, J
=7.5 Hz, 6 H). 13C NMR (101 MHz, (CD3)2SO) δ 140.59, 131.31,
128.75, 128.02, 127.76, 126.68, 126.60, 125.88, 21.56, 15.51. MS (EI)
m/z 300 (M+, 100%): 300, 285, 269, 203, 150.
4,5-Diethyl-2-mesityl-1H-imidazole (1): 2,4,6-Trimethylbenzal-
dehyde (2.00 g, 13.5 mmol), hexane-3,4-dione (8.2 mL, 67.6 mmol) and
ammonium acetate (52.00 g, 0.67 mol) were refluxed in glacial acetic
acid for 3 h after which the reaction mixture was cooled, diluted with
water and extracted twice with chloroform. Solvent was removed from
the combined organic extracts and the crude residue was purified on
silica gel column by using dichloromethane to remove residual hexane-
3,4-dione after which ethyl acetate was employed to elute ligand 1 as
colourless crystalline needles (Yield =1.2 g, 37%). Mp. 227 °C. Selected
Scheme 2. The studied monodentate azole N-donor ligand frameworks.
3

A.O. Eseola, et al.
MolecularCatalysis473(2019)110398
2-(Anthracen-9-yl)-4,5-diphenyl-1H-imidazole (3) and 2-(an-
4.8 mmol), aniline (0.52 g, 5.6 mmol) and (1.10 g, 14.3 mol).
Chromatography purification on silica gel was eluted by (i) 100%
chloroform to get the oxazole and then (ii) chloroform with increasing
amount of ethyl acetate.
thracen-9-yl)-1,4,5-triphenyl-1H-imidazole (4): Anthracene-9-car-
baldehyde (1.96 g, 9.5 mmol), benzil (2.00 g, 9.5 mmol), aniline
(1.06 g, 11.4 mmol) and ammonium acetate (2.20 g, 28.6 mmol).
Chromatography purification was by long silica gel column and 20:1
dichloromethane / ethyl acetate.
7 - (0.70 g, 31%) Mp. 176 °C. Selected IR peaks (ATR, cm−1): ν
3050 m (CeH aromatic), 1597(C]N imidazole), 1497vs, 1373vs,
753vs. 1H NMR (400 MHz, CDCl3) δ 8.97 (d, J =6.1 Hz, 1 H), 8.88 (d, J
=8.4 Hz, 1 H), 8.82 (d, J =8.2 Hz, 1 H), 8.50 (s, 1 H), 7.99 (dd, J = 5.8,
3.9 Hz, 2 H), 7.83 – 7.69 (m, 4 H), 7.63 – 7.56 (m, 1 H), 7.48 – 7.41 (m,
4 H), 7.36 – 7.24 (m, 4 H), 7.18 (t, J =7.4 Hz, 1 H), 7.11 (dd, J = 8.1,
6.7 Hz, 2 H). 13C NMR (101 MHz, CDCl3) δ 149.52, 137.51, 132.36,
130.87, 129.35, 129.14, 128.47, 128.40, 127.56, 127.48, 126.61,
3 - (0.4 g, 11%) Mp. 304 °C. Selected IR peaks (ATR, cm−1): ν
3053 m (CeH aromatic), 1625 m (C]C aromatic), 1443s, 733vs, 695vs.
1H NMR (400 MHz, (CD3)2SO) δ 12.96 (s, 1 H), 8.80 (s, 1 H), 8.23–8.17
(m, 2 H), 7.99–7.93 (m, 2 H), 7.69 (d, J =7.4 Hz, 2 H), 7.59 (dd,
J = 10.0, 5.6 Hz, 6 H), 7.44 (t, J =7.1 Hz, 2 H), 7.36 (t, J =7.1 Hz,
3 H), 7.28 (d, J =7.0 Hz, 1 H). 13C NMR (101 MHz, (CD3)2SO) δ
143.75, 137.42, 135.91, 131.60, 131.38, 131.29, 129.20, 128.88,
128.74, 128.43, 128.19, 128.05, 127.80, 127.08, 126.49, 126.05. MS
(EI) m/z 396 (M+, 100%): 396, 351, 168.
126.40, 125.92, 125.29, 124.19, 123.19, 121.13. MS (EI) m/z 470 (M+
,
100%): 470, 393, 266.
9 - (0.5 g, 26%) Mp. 308 °C. Selected IR peaks (ATR, cm−1): ν
3054 m (CeH aromatic), 1619 m (C]C aromatic), 1477vs, 1349vs,
775vs. 1H NMR (400 MHz, CDCl3) δ 8.87 (dd, J = 11.1, 4.8 Hz, 2 H),
8.81 – 8.76 (m, 1 H), 8.73 (s, 1 H), 8.39 – 8.35 (m, 1 H), 8.20 – 8.13 (m,
4 H), 7.84 – 7.81 (m, 1 H), 7.77 (m, 3 H), 7.59 – 7.53 (m, 4 H). 13C NMR
(101 MHz, CDCl3) δ 131.21, 128.69, 127.39, 125.60, 115.00. MS (EI)
m/z 395 (M+, 100%): 395, 365, 203, 163.
4 - (2.6 g, 58%) Mp. 237 °C. Selected IR peaks (ATR, cm−1): ν
3054 w (CeH aromatic), 1625 m (C]C aromatic), 1596s, 1494vs,
699vs. 1H NMR (400 MHz, CDCl3) δ 8.48 (s, 1 H), 7.99–7.96 (m, 2 H),
7.87 (d, J =8.0 Hz, 2 H), 7.76 (d, J =7.3 Hz, 2 H), 7.49 – 7.41 (m, 4 H),
7.30 (dd, J = 13.0, 6.1 Hz,8 H), 7.28 – 7.22 (m, 1 H), 6.85–6.77 (m,
4 H). 13C NMR (101 MHz, CDCl3) δ 145.11, 132.26, 131.02, 130.99,
129.86, 128.53, 128.43, 128.24, 127.58, 127.05, 126.45, 126.05,
125.21. MS (EI) m/z 472 (M+, 100%): 472, 395, 236, 165.
Preparation of palladium complexes
2-(Anthracen-9-yl)-1,4,5-tris(4-methoxyphenyl)-1H-imidazole
(5): Anthracene-9-carbaldehyde (0.76 g, 3.7 mmol), 1,2-bis(4-methox-
yphenyl)ethane-1,2-dione (1.00 g, 3.7 mmol), 4-methoxyaniline
(0.55 g, 4.4 mmol) and ammonium acetate (0.86 g, 11.1 mmol). Crude
product was purified by recrystallization in chloroform / hexane. (Yield
=1.2 g, 58%). Mp. 298 °C. Selected IR peaks (ATR, cm−1): ν 3062 w
(CeH aromatic), 2959 m (methyl), 1610s (C]C aromatic), 1513vs,
1245vs, 1029vs. 1H NMR (400 MHz, CDCl3) δ 8.46 (s, 1 H), 8.00–7.93
(m, 2 H), 7.83 (d, J =7.0 Hz, 2 H), 7.68 (d, J =8.2 Hz, 2 H), 7.48 – 7.39
(m, 4 H), 7.22–7.19 (m, 2 H), 6.85 (dd, J = 8.6, 6.4 Hz, 4 H), 6.74 (d, J
General procedure: A given amount of ligand and one or half
equivalent of either bis(acetonitrile)dichloridopalladium(II), bis(ben-
zonitrile)dichloridopalladium(II) or palladium acetate were weighed
into a glass vial. After addition of solvent, which was either acetonitrile,
ethanol or methanol, the vial is sealed and allowed to stand without
disturbance for at least overnight in order to allow self-assemble of the
preferred coordination product. The complexation product was filtered
off, washed with little amount of the utilized solvent and dried to obtain
solids of varying colour from yellow through orange to brown crystal-
line or amorphous materials. For several complexation experiments,
suitable single crystals were obtained and analysed for structural in-
formation. The individual synthetic and analytical data for all the iso-
lated coordination products are listed below. Each complexation ex-
periment was routinely monitored by microscope to optically observe
complete disappearance of ligand or palladium salt solids before fil-
tering the formed complex. Therefore, some of the complexation ex-
periments stood for several days before product filtration.
=8.9 Hz, 2 H), 6.32 (d, J =8.9 Hz, 2 H), 3.82 (s, 6 H), 3.50 (s, 3 H). 13
C
NMR (101 MHz, CDCl3) δ 132.27, 131.01, 130.90, 128.41, 128.12,
125.17, 118.14, 114.00, 113.66, 113.43, 55.23, 55.16, 54.99, 41.01.
MS (EI) m/z 562 (M+, 100%): 562, 455, 396, 281.
2-Mesityl-1-phenyl-1H-phenanthro[9,10-d]imidazole (6) and 2-me-
sitylphenanthro [9,10-d]oxazole (8): 2,4,6-Trimethylbenzaldehyde
(0.71 g, 4.8 mmol), phenanthrene-9,10-dione (1.00 g, 4.8 mmol), aniline
(0.53 g, 5.7 mmol) and (1.10 g, 14.3 mmol). Chromatography was done
on silica gel eluted by (i) 1:1 chloroform / n-hexane and (ii) 6:1 n-hep-
tane / ethyl acetate.
1-PdCl2MeCN:
4,5-Diethyl-2-mesityl-1H-imidazole
(48 mg,
0.20 mmol), PdCl2(MeCN)2 (26 mg, 0.20 mmol). (Yield =11 mg, 12%).
Mp. with decomposition > 300 °C Selected IR peaks (ATR, cm−1): ν
3179vs (N–H), 2972s (C–H alkyl), 1616s (C]C aromatic), 1436vvs,
850vs, 771vs. 1H NMR (300 MHz, (CD3)2SO) δ 12.41 (s, 1 H), 6.96 (s,
2 H), 2.97 (q, J =7.5 Hz, 2 H), 2.31 (s, 3 H), 2.11 (s, 6 H), 2.06 (s, 3 H),
1.43 (t, J =7.5 Hz, 3 H), 1.08 (m, 5 H). Anal. calc. for C18H25Cl2N3Pd:
C, 46.92; H, 5.47; N, 9.12%. Found: C, 46.97; H, 5.46; N, 9.07%.
12-PdCl2: 4,5-Diethyl-2-mesityl-1H-imidazole (48 mg, 0.20 mmol),
PdCl2(MeCN)2 (13 mg, 0.10 mmol). (Yield =21 mg, 32%). Mp. with
decomposition > 300 °C. Selected IR peaks (ATR, cm−1): ν 3283vs
(N–H), 2970vs (C–H alkyl), 1616vs (C]C aromatic), 1436vvs, 860vs,
693vs. 1H NMR (300 MHz, (CD3)2SO) δ 12.06 (s, 1 H), 11.98 (s, 1 H),
6.95 (s, 2 H), 6.79 (s, 2 H), 3.12 (dd, J = 14.0, 6.6 Hz, 2 H), 2.73 – 2.66
(m, 2 H), 2.43 (s, 3 H), 2.41 (m, 3 H), 2.32 (s, 3 H), 2.13 (s, 6 H), 1.82 (s,
6 H), 1.56 (t, J =7.5 Hz, 3 H), 1.12 – 1.00 (m, 10 H). Anal. calc. for
6 - (0.6 g, 30%) Mp. 216 °C. Selected IR peaks (ATR, cm−1): ν
2920 w (CeH alkyl), 1611s (C]C aromatic), 1496vs, 1452vs, 1376vs,
697vs. 1H NMR (400 MHz, (CD3)2SO) δ 8.93 (d, J =8.3 Hz, 1 H), 8.88
(d, J =8.3 Hz, 1 H), 8.63 (dd, J = 7.9, 1.2 Hz, 1 H), 7.79 – 7.73 (m,
1 H), 7.68 (ddd, J = 8.4, 7.2, 1.4 Hz, 1 H), 7.60 – 7.46 (m, 6 H), 7.37 –
7.31 (m, 1 H), 7.08 (dd, J = 8.3, 0.8 Hz, 1 H), 6.86 (s, 2 H), 2.21 (s,
3 H), 2.06 (s, 6 H). 13C NMR (101 MHz, (CD3)2SO) δ 151.30, 139.03,
138.20, 137.79, 137.20, 130.29, 130.18, 128.77, 128.07, 128.04,
127.94, 127.87, 127.29, 127.04, 126.86, 126.08, 125.59, 124.92,
124.10, 122.91, 122.57, 120.78, 21.18, 20.36. MS (EI) m/z 412 (M+
,
100%): 412, 397, 335, 266, 206.
8 - (0.6 g, 37%) Mp. 313 °C. Selected IR peaks (ATR, cm−1): ν
3069 w (CeH aromatic), 2962 m (CeH alkyl), 1614s (C]C aromatic),
1468vs, 743vs, 721vs. 1H NMR (400 MHz, (CD3)2SO) δ 9.00 (t, J
=9.1 Hz, 2 H), 8.50 (dd, J = 7.8, 1.2 Hz, 1 H), 8.33 – 8.28 (m, 1 H),
7.86 – 7.76 (m, 4 H), 7.12 (s, 2 H), 2.37 (s, 3 H), 2.34 (s, 6 H). 13C NMR
(101 MHz, (CD3)2SO) δ 161.99, 155.18, 140.62, 138.62, 137.91,
134.44, 131.54, 129.18, 129.09, 128.84, 128.43, 127.52, 127.02,
125.90, 124.79, 124.62, 122.79, 121.17, 121.03, 120.79, 120.13,
21.33, 20.62. MS (EI) m/z 337 (M+, 100%): 337, 321, 308, 294, 163.
2-(Anthracen-9-yl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (7)
and 2-(anthracen-9-yl)phenanthro[9,10-d]oxazole (9): Anthracene-9-
carbaldehyde (0.98 g, 4.8 mmol), phenanthrene-9,10-dione (1.00 g,
C
32H44Cl2N4Pd: C, 58.05; H, 6.70; N, 8.46%. Found: C, 58.12; H, 6.83;
N, 8.49%.
12-Pd(OAc)2:
4,5-Diethyl-2-mesityl-1H-imidazole
(48 mg,
0.20 mmol), Pd(OAc)2 (22 mg, 0.10 mmol). (Yield =43 mg, 61%). Mp.
311 °C. Selected IR peaks (ATR, cm−1): ν 2969s (C–H alkyl), 1618 m
(C = C aromatic), 1588vs (C = N), 1386vs, 1311vs, 853 s, 700 s. 1H
NMR not obtained due to poor solubility. Anal. calc. for C36H50N4O4Pd:
C, 60.97; H, 7.11; N, 7.90. Found: C, 60.79; N, 6.99; N, 7.77.
2-PdCl2MeCN: 2-(Anthracen-9-yl)-4,5-diethyl-1H-imidazole (60 mg,
4

A.O. Eseola, et al.
MolecularCatalysis473(2019)110398
0.20 mmol), PdCl2(MeCN)2 (52 mg, 0.20 mmol). (Yield =75 mg, 73%).
Mp. 271 °C. Selected IR peaks (ATR, cm−1): ν 3245vs (N–H), 3060 m
(CeH aromatic), 2926 m (CeH alkyl), 1617 m (C = C aromatic),
1402vs, 736vs. 1H NMR (300 MHz, (CD3)2SO) δ 12.91 (s, 1 H), 8.86 (s,
1 H), 8.18 (d, J =8.4 Hz, 2 H), 7.66 (d, J =8.8 Hz, 2 H), 7.52 (dddd,
J = 9.9, 7.9, 6.6, 1.2 Hz, 4 H), 3.13 (q, J =7.4 Hz, 2 H), 2.65 (q, J
=7.6 Hz, 2 H), 2.06 (s, 6H, more acetonitrile then 3H expected), 1.56
746vs. 1H NMR (300 MHz, (CD3)2SO) δ 11.12 (d, J =7.3 Hz, 1 H), 8.99
(d, J =8.3 Hz, 2 H), 8.93 (d, J =8.3 Hz, 1 H), 8.88 (d, J =8.1 Hz, 1 H),
8.61 (dd, J = 7.8, 1.3 Hz, 1 H), 8.02 (t, J =7.6 Hz, 1 H), 7.83 (t, J
=7.2 Hz, 1 H), 7.75 (t, J =7.0 Hz, 1 H), 7.67 (dd, J = 12.6, 4.5 Hz,
2 H), 7.35 (q, J =7.4 Hz, 2 H), 7.07 (d, J =7.6 Hz, 1 H), 6.91 (s, 2 H),
6.85 (s, 2 H), 2.23 (t,
J =6.8 Hz, 12 H). +Anal. calc. for
C
32H27Cl2N3Pd.1½H2O: C, 58.42; H, 4.60; N, 6.39%. Found: C, 58.70;
(t,
J =7.5 Hz, 3 H), 1.20 (t, J =7.5 Hz, 3 H). Anal. calc. for
H, 4.12; N, 6.65%.
C
23H23Cl2N3Pd.1/3MeCN: C, 53.39; H, 4.54; N, 8.77%. Found: C, 53.04;
6-PdCl2PhCN: 2-Mesityl-1-phenyl-1H-phenanthro[9,10-d]imidazole
(20 mg, 0.05 mmol), PdCl2(PhCN)2 (16 mg, 0.05 mmol). (Yield
=26 mg, 75%). Mp. 246 °C. Selected IR peaks (ATR, cm−1): ν 3059 w
(C–H aromatic), 2918 m (C–H alkyl), 1614s (C = C aromatic), 1494vs,
1024s, 755vs. 1H NMR (300 MHz, (CD3)2SO) δ 7.76 – 7.67 (m, 1 H),
7.68 – 7.63 (m, 1 H), 7.61 – 7.53 (m, 1 H), 7.35 (q, J =8.0 Hz, 1 H),
7.06 (d, J =7.4 Hz, 1 H), 6.91 (s, 1 H), 6.85 (s, 1 H), 2.21 (s, 1 H), 2.05
(s, 1 H). Anal. calc. for C37H29Cl2N3Pd.PhCN.2½H2O: C, 62.83; H, 4.67;
N, 6.66%. Found: C, 62.98; H4.32; N, 6.66%.
H, 4.45; N, 8.62%.
22-PdCl2: 2-(Anthracen-9-yl)-4,5-diethyl-1H-imidazole (60 mg,
0.20 mmol), PdCl2(MeCN)2 (26 mg, 0.10 mmol). (Yield =63 mg, 81%).
Mp. with decomposition 362 °C. Selected IR peaks (ATR, cm−1): ν
3257vs (N–H), 3051 m (C–H aromatic), 2970s (C–H alkyl), 1613s
(C = C aromatic), 1445vs, 735vs, 607vs. 1H NMR (300 MHz, (CD3)2SO)
δ 12.44 (s, 2 H), 8.81 (s, 2 H), 8.16 (d, J =8.4 Hz, 4 H), 7.64 – 7.48 (m,
8 H), 7.41 – 7.32 (m, 4 H), 2.42 – 2.32 (m, 4 H), 2.25 (q, J =7.1 Hz,
4 H), 0.99 (t, J =7.5 Hz, 6 H), 0.26 (t, J =7.4 Hz, 6 H) (Ethanol seen in
NMR). Anal. calc. for C42H40Cl2N4Pd.4EtOH: C, 62.40; H, 6.70; N,
5.82%. Found: C, 62.11; H, 6.30; N, 5.89%.
62-PdCl2:
2-Mesityl-1-phenyl-1H-phenanthro[9,10-d]imidazole
(41 mg, 0.10 mmol), PdCl2(MeCN)2 (13 mg, 0.05 mmol) in ethanol.
(Yield =48 mg, 96%). Mp. 373 °C. Selected IR peaks (ATR, cm−1): ν
3529 m, 3054 m (C–H aromatic), 2973 m (C–H methyl), 1605 m (C = C
aromatic), 1447vs, 1038vs, 697vs. Complex NMR spectra in di-
methylsulphoxide was obtained, but signals for methyl and aromatic
protons were identified. Anal. calc. for C60H48Cl2N4Pd.EtOH: C, 71.89;
H, 4.83; N, 5.59%. Found: C, 71.03; H, 5.19; N, 5.34%.
22-Pd(OAc)2: 2-(Anthracen-9-yl)-4,5-diethyl-1H-imidazole (60 mg,
0.20 mmol), PdCl2(OAc)2 (22 mg, 0.10 mmol). (Yield =34 mg, 41%).
Mp. 281 °C. Selected IR peaks (ATR, cm−1): ν 3633 s 2973s (C–H alkyl),
1611s (C]C aromatic), 1577vs (C]N), 1374vs, 1328vs, 731vs. 1H
NMR (300 MHz, (CD3)2SO) δ 12.30 (s, 2 H), 8.74 (s, 2 H), 8.13 (d, J
=8.4 Hz, 4 H), 7.72 (d, J =8.6 Hz, 4 H), 7.52 – 7.46 (m, 4 H), 7.37 –
7.31 (m, 4 H), 2.39 (dd, J = 14.2, 7.0 Hz, 8 H), 1.02 (t, J =7.5 Hz, 6 H),
7-PdCl2MeCN: 2-(Anthracen-9-yl)-1-phenyl-1H-phenanthro[9,10-d]
imidazole (47 mg, 0.10 mmol), PdCl2(MeCN)2 (26 mg, 0.05 mmol).
(Yield =44 mg, 64%). Mp. 388 °C. Selected IR peaks (ATR, cm−1): ν
3059 m (C–H aromatic), 2926 m, 1613s (C = C aromatic), 1452vs,
722vs, 711vs. 1H NMR (300 MHz, (CD3)2SO) δ 9.08 – 8.90 (m, 2 H),
8.80 – 8.61 (m, 2 H), 8.14 – 8.00 (m, 2 H), 7.90 – 7.31 (m, 12 H), 7.29 –
6.85 (m, 4 H), 2.06 (s, 3 H) (acetonitrile methyl signal is in ratio 1:1
with aromatic protons). Anal. calc. for C37H25Cl2N3Pd: C, 64.50; H,
3.66; N, 6.10. Found: C, 64.11; H, 3.57; N, 6.13.
0.70 (s, 6 H), 0.58 (t,
J
=6.8 Hz, 6 H). Anal. calc. for
C
36H50N4O4Pd.2½H2O: C, 63.48; H, 5.91; N, 6.44. Found: C, 63.42; H,
5.77; N, 6.78.
3-PdCl2MeCN:
2-(Anthracen-9-yl)-4,5-diphenyl-1H-imidazole
(39 mg, 0.10 mmol), PdCl2(MeCN)2 (26 mg, 0.10 mmol). (Yield
=45 mg, 73%). Mp. 329 °C. Selected IR peaks (ATR, cm−1): ν 3731 s,
3054 w (C–H aromatic), 2973s (C–H alkyl), 1577vs (C = N), 1446vs,
1330vs, 734vs. 1H NMR (300 MHz, (CD3)2SO) δ 14.04 (s, 1 H), 9.00 (s,
7-PdCl2PhCN: 2-(anthracen-9-yl)-1-phenyl-1H-phenanthro[9,10-d]
imidazole (20 mg, 0.05 mmol), PdCl2(PhCN)2 (16 mg, 0.10 mmol) in
methanol. (Yield =31 mg, 82%). Mp. 388 °C. Selected IR peaks (ATR,
cm−1): 3057 s (C–H aromatic), 1615 m (C = C aromatic), 1521s,
1494vs, 707vs. 1H NMR (300 MHz, (CD3)2SO) δ 9.00 (d, J =8.3 Hz,
1 H), 8.95 (d, J =7.7 Hz, 1 H), 8.71 (s, 1 H), 8.65 (dd, J = 7.8, 1.5 Hz,
1 H), 8.14–8.06 (m, 3 H), 7.88 – 7.80 (m, 3 H), 7.78–7.67 (m, 5 H),
7.63–7.32 (m, 14 H), 7.28–7.06 (m, 5 H). Anal. calc. for
1 H), 8.39
– 7.29 (m, 18 H), 2.06 (s, 3 H). Anal. calc. for
C
31H23Cl2N3Pd.H2O: C, 58.83; H, 3.98; N, 6.64%. Found: C, 58.78; H,
3.95; N, 6.39%.
32-PdCl2: 2-(Anthracen-9-yl)-4,5-diphenyl-1H-imidazole (39 mg,
0.10 mmol), PdCl2(MeCN)2 (13 mg, 0.05 mmol) in ethanol. (Yield
=41 mg, 85%). Mp. 347 °C. Selected IR peaks (ATR, cm−1): ν 3532 s
(N–H), 3054 w (C–H aromatic), 2975 m (C–H alkyl), 1594s (C = N),
1447vs, 1074vs, 736vs. 1H NMR (300 MHz, (CD3)2SO) δ 13.39 (s, 2 H),
8.85 (s, 2 H), 8.31 – 6.09 (m, 36 H) (Ethanol peaks present). Anal. calc.
for C58H40Cl2N4Pd.2H2O.EtOH: C, 68.48; H, 4.79; N, 5.32%. Found: C,
68.20; H, 4.82; N, 5.12%.
C
42H27Cl2N3Pd.PhCN.3H2O: C, 64.80; H, 4.22; N, 6.17%. Found: C,
64.78; H, 3.52; N, 6.44%.
8-PdCl2MeCN: The ligand 2-mesitylphenanthro[9,10-d]oxazole
(33 mg, 0.10 mmol), PdCl2(MeCN)2 (26 mg, 0.10 mmol). (Yield
=55 mg, 80%). Mp. 260 °C. Selected IR peaks (ATR, cm−1): ν 2973 m
(CeH methyl), 1612s (C = C aromatic), 1573s, 756vs, 746vs. 1H NMR
(300 MHz, (CD3)2SO) δ 8.98 (dd, J = 8.6, 6.8 Hz, 2 H), 8.48 (dd,
J = 7.7, 1.5 Hz, 1 H), 8.32 – 8.26 (m, 1 H), 7.86 – 7.69 (m, 4 H), 7.10 (s,
2 H), 2.35 (s, 3 H), 2.32 (s, 6 H), 2.04 (s, 3 H) (the acetonitrile proton
4-PdCl2MeCNcis: 2-(Anthracen-9-yl)-1,4,5-triphenyl-1H-imidazole
(47 mg, 0.10 mmol), PdCl2(MeCN)2 (26 mg, 0.10 mmol). (Yield
=55 mg, 80%). Mp. 283 °C. Selected IR peaks (ATR, cm−1): ν 3530 m,
3054 m (C–H aromatic), 1625 m (C]C aromatic), 1445vs, 764vs,
695vs. 1H NMR (300 MHz, (CD3)2SO) δ 8.82 (s, 1 H), 8.37 – 6.76 (m,
23 H), 2.06 (s, 3 H) (Excess acetonitrile signal is observed). Anal. calc.
for C37H27Cl2N3Pd.½MeCN: C, 64.15; H, 4.04; N, 6.89%. Found: C,
63.83; H, 3.90; N, 6.68%.
signal
is
more
then
1
equivalent).
Anal.
calc.
for
C
26H22Cl2N2OPd.½MeCN: C, 56.27; H, 4.11; N, 6.08%. Found: C,
56.21; H, 4.07; N, 6.41%.
5-PdCl2MeCN: 2-(Anthracen-9-yl)-1,4,5-tris(4-methoxyphenyl)-1H-
imidazole (56 mg, 0.10 mmol), PdCl2(MeCN)2 (26 mg, 0.10 mmol).
(Yield =34 mg, 44%). Mp. > 400 °C. Selected IR peaks (ATR, cm−1): ν
3529 m, 3054 m (C–H aromatic), 2974 m (C–H methyl), 1625 m (C = C
aromatic), 1412s, 1028s, 736vs, 697vs. 1H NMR (300 MHz, (CD3)2SO)
δ 8.83 (s, 1 H), 8.32 – 6.34 (m, 20 H), 3.87 – 3.60 (m, 6 H), 3.45 – 3.38
(m, 3 H). Anal. calc. for C40H33Cl2N3O3Pd.H2O: C, 60.13; H, 4.42; N,
5.26%. Found: C, 59.94; H, 4.03; N, 4.74%.
9-PdCl2MeCN:
2-(Anthracen-9-yl)phenanthro[9,10-d]oxazole
(39 mg, 0.10 mmol), PdCl2(MeCN)2 (26 mg, 0.10 mmol). (Yield
=43 mg, 70%). Mp. 340 °C. Selected IR peaks (ATR, cm−1): ν 3630 m,
3054 m (C–H aromatic), 1607 m (C = C aromatic), 1452vs, 1031vs,
751vs, 721vs. 1H NMR (300 MHz, (CD3)2SO) δ 9.09 – 9.03 (m, 2 H),
9.00 (s, 1 H), 8.61 – 8.57 (m, 1 H), 8.34 – 8.25 (m, 3 H), 8.06 (d, J
=9.8 Hz, 2 H), 7.85 (ddd, J = 12.1, 6.0, 3.7 Hz, 4 H), 7.68 – 7.58 (m,
4 H), 2.06 (s, 3 H) (acetonitrile signal shows more than 1 equivalent).
Anal. calc. for C31H20Cl2N2OPd.MeCN: C, 60.52; H, 3.54; N, 6.42%.
Found: C, 61.71; H, 3.49; N, 6.25.
6-PdCl2MeCN:
2-Mesityl-1-phenyl-1H-phenanthro[9,10-d]imida-
zole (41 mg, 0.10 mmol), PdCl2(MeCN)2 (26 mg, 0.10 mmol). (Yield
=51 mg, 81%). Mp. 395 °C. Selected IR peaks (ATR, cm−1): ν 3056 w
(C–H aromatic), 2920 m (C–H methyl), 1614s (C]C aromatic), 1453vs,
5

A.O. Eseola, et al.
MolecularCatalysis473(2019)110398
Scheme 3. (a) Complexation scheme, (b) isolated mono-ligand complexes 1-PdCl2MeCN – 9-PdCl2MeCN and (c) isolated bis-ligand and cyclometallated palladium
(II) complexes 12-PdCl2, 12-Pd(OAc)2, 22-PdCl2, 22-Pd(OAc)2, 32-PdCl2 and 62-PdCl2.
6

A.O. Eseola, et al.
MolecularCatalysis473(2019)110398
Structure determinations
Results and discussion
The intensity data for the compounds were collected on a Nonius
KappaCCD diffractometer using graphite-monochromated Mo-Kα ra-
diation. Data were corrected for Lorentz and polarization effects; ab-
sorption was taken into account on a semi-empirical basis using mul-
tiple-scans [57–60]. The structures were solved by direct methods
(SHELXS) and refined by full-matrix least squares techniques against
Fo2 (SHELXL-97) [60]. The hydrogen atoms bonded to the amine of the
compounds 12-PdCl2, 12-Pd(OAc)2, 22-PdCl2, 22-Pd(OAc)2 and
PdIIP2Q2 and bonded to the hydroxyl-group O2 of PdIIP2Q2 were lo-
cated by difference Fourier synthesis and refined isotropically. All other
hydrogen atoms were included at calculated positions with fixed
thermal parameters. All non-hydrogen and non-disordered atoms were
refined anisotropically [60]. The program XP [61] was used for struc-
ture representations. Crystallographic data as well as structure solution
and refinement details are summarized in Supporting information Table
S1.
Syntheses and characterizations
Condensation of aldehydes and α-diketones yielded the imidazole or
oxazole ligands 1 – 9 in good yields (Scheme 2). The ligands were
systematically designed to possess varied electronic N-donor strengths,
substituent bulkiness and rigidity around the azole N-donor. Com-
plexation reactions were achieved by allowing a given ligand and a
given palladium(II) salt to stand and self-assemble in a given solvent
(Scheme 3(a)). Analytical characterization data for the prepared ligands
and complexes are in conformity with expected values (see also Sup-
plementary information S1 for individual 1H and 13C NMR spectra). 1
H
NMR of the complexes were only measured in deuterated di-
methylsulphoxide since solubility in other deuterated solvents is diffi-
cult. Thus, the strongly coordinating property of dimethylsulphoxide,
which has been structurally evidenced in our previous report [49], is
responsible for why complicated nmr spectra with more than one set of
ligand proton signals is observed for some of the complexes (see Sup-
plementary information S2).
Catalysis experiments
In the complexation reaction, either of PdCl2(MeCN)2,
PdCl2(PhCN)2, PdCl2(COD), (Pd0)2(dba)3 and Pdn(OAc)2n were in-
dividually experimented as reactant and the isolated series of palladium
complexes are presented in Scheme 3(b) and (c). The desired mono-
azole mono-solvent complexes 1-PdCl2MeCN – 9-PdCl2MeCN (Scheme
3(b)) were only obtained in acetonitrile and from the chloride starting
palladium salts while the bis-imidazole complexes 12-PdCl2, 12-Pd
(OAc)2, 22-PdCl2, 22-Pd(OAc)2, 32-PdCl2 and 62-PdCl2 (Scheme 3(c)),
which were also isolated and studied for comparative purposes, are
formed in the absence of acetonitrile as solvent media or when palla-
dium acetate is used even if in acetonitrile.
In routine catalyses experiments, substrates were coupled in sealed
glass vials maintained at 45 °C by thermostated oil baths equipped with
magnetic stirrers. Some higher temperature runs were made. Boronic
acid (1.2 mmol) and aryl halide reagents (1 mmol), base additive (1.2
eq.) and catalyst material (0.2 mol %) were stirred in 4 ml water /
ethanol mixture (1:3). After the desired duration, an aliquot of the re-
sultant reaction mixture is transferred to a clean vial. The solvent
mixture is evaporated under vacuum pressure with liquid nitrogen
cooling trap in place. The residue, which is collected in deuterated di-
methylsulphoxide and transferred into an NMR tube, is subjected to
proton NMR analysis. Yields were routinely estimated by comparison of
the NMR signals of corresponding functions present on both the ar-
ylhalide and the coupled biphenyl. The biphenyl 1-(4′-(hydro-
xymethyl)-[1,1′-biphenyl]-4-yl)ethan-1-one has been previously iso-
lated and characterized [44]. New biphenyl species were also analysed:
4′-methoxy-[1,1′-biphenyl]-2-amine: EI MS: m/z 199 [M]+ (100%,
calc. 199.25), 184 (-methyl). 4-(4-methoxyphenyl)pyridine: EI MS: m/z
185 [M]+ (100%, calc. 185.23), 108 (anisyl), 78 (pyridyl). 4,4′-di-
methoxy-[1,1′-biphenyl]-2-carboxylic acid: EI MS: m/z 258 [M]+
(100%, calc. 258.27), 241 (– hydroxy), 213 (– carboxy), 152, 106
(anisyl). 4,4′-dimethoxy-3-nitro-1,1′-biphenyl: EI MS: m/z 259 [M]+
(100%, calc. 259.26), 244 (-methyl), 214 (-nitro), 198 (– methyl and
nitro), 183 (– 2 methyl and nitro), 168. (3′,5′-dimethyl-[1,1′-biphenyl]-
4-yl)methanol: EI MS: m/z 212 [M]+ (100%, calc. 212.29), 195 (–
hydroxy), 183 (– 2methyl). 4-(benzyloxy)-4′-methoxy-1,1′-biphenyl: EI
MS: m/z 290 [M]+ (100%, calc. 290.36), 199 (– benzyl), 91 (– benzyl).
It is noteworthy that only mono-ligand complexes were obtained
from some ligands with bulky and rigid substituents (e.g. anthraceneyl
for ligands 4 and 7), but both mono-ligand and bis-ligand complexes
were obtainable from ligands with smaller substituent sizes around the
azole N-donor (e.g. 4,5-diethyl of 1 and 2 or 2-mesityl of ligands 1 and
6). This observation highlights the importance of bulkier substituents
and the consequent ligand-ligand repulsion on preferential mono-ligand
complexation.
Reactivity of Pd0 towards oxidative decomposition and 1,2-phenyl ring shift
of ligand 3
A interesting reaction in which ligand transformation is catalysed by
a Pd0 reagent was encountered during a complexation experiment
whereby ligand 3 and Pd2(dba)3 were allowed to stand for weeks in
ethanol. An isolated crystalline palladium product (PdIIP2Q2) revealed
that the Pd0 reagent facilitated a novel oxidative 1,2-phenyl ring re-
arrangement on the 4,5-imidazole carbon positions as well as formation
of further oxidative decomposition products of the ligand to yield 2-
Ligand donor strength evaluation
(anthracen-9-yl)-5,5-diphenyl-1,5-dihydro-4H-imidazol-4-one
(P),
amino(anthracen-9-yl)methanol (Q) and anthracene-9,10-dione (R)
(Scheme 4, Fig. 1). The products P and Q are coordinated to a central
PdII while the product R is incorporated in a 1D chain by hydrogen
bonding and π-π stacking (Fig. 1). It is also reasonable to envisage that
more oxidation products probably exist in the reaction mixture.
Therefore, it could be concluded that deployment of such Pd0 reagent in
catalysis would be capable of much more reactivities than awaited even
at room temperature, which would lead to complex by-products profile
and limited success of the target reaction. A further study of potential
utility of this 1,2-phenyl ring shift is considered for further study.
Protonation equilibrium constant evaluations were spectro-
scopically determined via UV–vis or fluorescence measurements using
10−5 M solutions of individual ligand in 7:3 ethanol / water mixture.
Potassium chloride was applied to aid constant ionic strength of 0.1 M.
Concentrated potassium hydroxide or hydrochloric acid solutions in
negligible volumes were added to adjust pH values, which was mon-
itored with the aid of Mettler Toledo MPc227 pH/Conductivity Meter
calibrated by pH 4.0, 7.0 and 10.1 buffers. The pKa values were ob-
tained using Origin software by non-linear least square fittings made to
the plots of pH versus absorbance at given wavelengths and R2 values
were generally in the range 0.990 – 0.999.
X-ray structural analyses of the studied palladium precatalysts
Single crystal structural analyses confirmed identities of the syn-
thetized compounds. Fig. 2 presents structures of the mono-ligand
7

A.O. Eseola, et al.
MolecularCatalysis473(2019)110398
Scheme 4. Oxidative decomposition and a novel 1,2-phenyl ring shift on the 4,5-imidazole carbons of ligand 3 mediated by Pd2(dba)3.
mono-solvent dichloridopalladium(II) complexes 4-PdCl2MeCNcis
(monoclinic 21/c), 6-PdCl2MeCN (orthorhombic 21), 6-
the mesityl analogues (see ΔPd–Cl for 3-PdCl2MeCN, 7-PdCl2MeCN and
9-PdCl2MeCN compared to ΔPd–Cl for 6-PdCl2MeCN, 6-PdCl2PhCN and
8-PdCl2MeCN; Table 2). Therefore, it could be suggested that this steric
effect by the anthraceneyl moiety may have contributed to the eventual
cis-coordination by ligand 4.
P
P c a
PdCl2PhCN (monoclinic C2/c) and 8-PdCl2MeCN (triclinic P ī). The
Ortep plots for the bis-ligand analogues 12-PdCl2 (tetragonal I 41/a), 12-
Pd(OAc)2 (tetragonal I 41/a), 22-PdCl2 (orthorhombic P b c a) and 22-
Pd(OAc)2 (triclinic P ī) are also displayed in Fig. 3. The crystal data and
refinement details for these complexes are summarized in Supporting
Information Table S1 while selected bond lengths are collected in
Table 2 (see Supplementary information Table S2 for the complete table
of bond lengths). It is observable that the Pd–N bond lengths for the bis-
ligand complexes 12-PdCl2, 22-PdCl2, 12-Pd(OAc)2 and 22-Pd(OAc)2 are
longer than for the mono-ligand complexes 3-PdCl2MeCN, 4-
PdCl2MeCNcis, 6-PdCl2MeCN, 6-PdCl2PhCN, 7-PdCl2MeCN, 8-
PdCl2MeCN and 9-PdCl2MeCN, which implies ligand-ligand repulsions
in the bis-ligand complexes.
N-Donor strengths estimation for ligands 1–9
The N-donor strength of ligands 1 to 9 were determined for study of
correlation between ligand electronic properties and eventual trends of
catalyst activities. The donor strengths, which were derived as pKa
values, were estimated by spectroscopic method using the
Henderson–Hasselbalch theory [62] (Eq. (1); A0 and Af are wavelength
dependent absorbance values for the solute molecules in the fully
protonated or deprotonated forms respectively while A is absorbance
for mixtures of these species anywhere in between the two extremes of
the protonation-deprotonation equilibrium).
Furthermore, among all analysed crystals, the cis-di-
chloridopalladium(II) coordination environment observed in the
structure of 4-PdCl2MeCNcis is noteworthy. While all the mono-ligand
complexes 1-PdCl2MeCN – 9-PdCl2MeCN were prepared from the trans-
dichlorido compound PdCl2(MeCN)2, repeats of these preparations
using ligands 1 – 9 and the cis-dichlorido starting material PdCl2(COD)
were deliberately carried out in anticipation of possible cis-coordination
product (Scheme 3(a)). Nevertheless, single crystal analyses of all the
complexes formed in the repeat reactions always reproduced the same
unit cell parameter outcomes as for the products obtained using
PdCl2(MeCN)2. It was therefore concluded that, regardless of the trans-
or cis-dichloridopalladium(II) starting materials, ligand coordination
preference could be observed. Ligand 4 inherently and reproducibly
formed the cis-dichlorido product 4-PdCl2MeCNcis (Fig. 2) while others
reproduced the trans-dichlorido structures. Furthermore, the difference
between Pd–Cl lengths within a given complex (i.e. ΔPd–Cl) is sig-
nificantly larger for the anthraceneyl-substituted complexes relative to
A
Af
Ao
A
pKa = pH + log
(1)
In addition to UV–vis spectroscopy, fluorescence measurements
were used as intervention [63–65] for ligands with poor absorption
spectral profiles. Fig. 4 presents examples of stacked absorption or
emission spectra showing changes under varying pH conditions (see
Supplementary information S5 for the spectral stack of ligands 1–9).
The insets show sigmoid fitting on plots of absorbance or emission in-
tensities against pH with R2 values generally higher than 99%.
The estimated pKa values are presented along with their respective
ligand structures in Table 3. Donor strengths of the ligands follow the
trend 1 > 2 > 3 > 4 > 6 > 7 > 9 > 8. It is interesting that the imi-
dazole N-donor strength could be manipulated to reach high levels as
for phosphines (e.g. pKa for PPh3 = 7.64) [66]. It is also observable that
Fig. 1. Ortep plot of PdIIP2Q2, which proves the oxidative decomposition and 1,2-shift of 4-phenyl ring of ligand 3 caused by Pd2(dba)3. Ellipsoids are drawn at the
20% probability level and some proton and solvent molecules have been omitted for clarity.
8

A.O. Eseola, et al.
MolecularCatalysis473(2019)110398
Fig. 2. Orteop plots of the isolated bis-ligand complexes 4-PdCl2MeCNcis, 6-PdCl2MeCN, 6-PdCl2PhCN and 8-PdCl2MeCN drawn at the 50% probability level. Some
protons and solvents of chrystallization have been omitted for clarity.
the alkyl-substituent moieties supported higher N-donor strengths
while the aromatic and fused aromatic substituents enabled lower N-
donor strengths. For instance, highest donor strengths observed for li-
gands 1 and 2 is attributable to the electron-releasing influence of 4,5-
diethyl-imidazole substitution. The methyl-substituents of the mesityl
group also contributed to increasing imidazole N-donor strengths (e.g.
ligand 1 vs 2; ligand 6 vs 7). Conversely, the electron-withdrawing-
effect of the oxazole oxygen lead to the lowest N-donor capacities. The
pKa value for ligand 5 could not be evaluated owing to existence of
more than one protonation-deprotonation equilibria through the
methoxy functions. The observed pKa trend reveals an organised elec-
tronic donor strength variation in the ligand series.
mixture (4 ml of 1:3), 45 °C, 15 min). Preliminary catalytic runs show
very encouraging results at ambient temperatures and within short
reaction times relative to other known results for N-donor ligands
supported palladium species (Table 1) [48]. Therefore, in order to en-
able comparison for ligands 1–9, catalysis temperature was restricted to
45 °C while the stirring time was also restricted to 15 min. The out-
comes are summarized in Table 4.
For the in situ formation of catalyst species, the observed trend of
turnover frequencies (TOFs) reveals a strong dependence on ligand
donor strengths and some dependence on substituent bulkiness
(Table 4). TOF values mainly increased with decreasing ligand donor
strengths. For instance, the lower donor strength ligands 6 (pKa = 3.0)
and 8 (pKa = 0.9) respectively yielded 67% (TOF =1340 h−1) and 62%
(TOF =1240 h−1) at the ambient conditions of 45 °C and 15 min. Such
performance is particularly worthy of note for N-donor ligand systems.
On the other hand, ligands 1 (pKa = 8.5) and 2 (pKa = 7.3), which have
the highest donor strengths in the studied series, respectively yielded
only 18% (TOF =360 h−1) and 8% (TOF =160 h−1), which is com-
parable to the 17% (TOF =340 h−1) recorded for triphenylphosphine
(PPh3, pKa [66] = 7.6, Table 4). It was also observed that the anthra-
cene-substituted ligand analogues consistently performed poorer than
their corresponding mesityl analogues, which indicates that dis-
advantages could result from excessive substituents bulkiness (i.e.
Table 4, ligand 2 vs.1, ligand 7 vs 6 and ligand 9 vs 8).
In situ catalyst generation by ‘PdCl2(MeCN)2 + ligand’ approach
Design of affordable N-donor ligand backbones capable of sup-
porting efficient generation of palladium active species is a key purpose
of this study. Consequently, using the in situ ‘PdCl2(MeCN)2 + ligand’
catalyst generation approach under the same reaction settings ac-
cording to a previously established reaction procedure [43], influence
of the monodentate N-donor ligands 1 – 9 on palladium-catalysed CeC
coupling were compared (reaction: (4-bromophenyl)methanol
(1 mmol), (4-acetylphenyl)boronic acid (1.2 mmol), K2CO3 (1.5 mmol),
PdCl2(MeCN)2 (0.2 mol %), ligands (0.4 mol %), in water / ethanol
9

A.O. Eseola, et al.
MolecularCatalysis473(2019)110398
Fig. 3. Orteop plots of the isolated bis-ligand complexes 12-PdCl2, 12-Pd(OAc)2, 22-PdCl2 and 22-Pd(OAc)2 drawn at the 30% probability level. Some protons and
solvents of chrystallization have been omitted for clarity.
reactions. As presented in Table 5, comparing temperature variation
while other parameters remain unchanged shows that TOF values in-
creases from 700 h−1 at 20 °C to 1960 h−1 at 70 °C. It was concluded to
use 45 °C for further reactions.
Table 2
Selected bond lengths around the coordination environments of palladium for
obtained crystal structuresa.
Entry
Catalyst
Pd–NL
Pd–NS
Pd–Cl
Pd–Cl
ΔPd–Cl
Negligible catalyst activity is observed in the absence of the additive
base while replacement of K2CO3 by K3PO4 did not yield meaningful
difference. Under microwave condition, 1-minute reaction maintained
at 45 °C produced 54% yield (TOF = 16,200 h−1) while using water
alone as solvent gave slightly higher yield (89%, TOF =1780 h−1) than
in the ethanol / water mixture (74%, TOF =1480 h−1). However, for
comparing catalyst trends among the synthesized complexes, it was
concluded that further reactions would be conducted in water / ethanol
mixture and without microwave assistance in order to minimize influ-
ence of precatalyst solubility differences and microwave effects on
precatalysts comparisons.
1
2
3
4
5
6
7
3-PdCl2MeCN
4-PdCl2MeCNcis
6-PdCl2MeCN
6-PdCl2PhCN
7-PdCl2MeCN
8-PdCl2MeCN
9-PdCl2MeCN
2.000
2.017
2.015
2.000
1.996
2.005
2.000
Pd–NL
2.017
2.019
1.990
2.003
1.995
1.992
2.001
1.982
1.990
Pd–NL
2.017
2.019
2.303
2.294
2.293
2.289
2.304
2.288
2.302
Pd–Cl
2.315
2.301
2.282
2.275
2.293
2.290
2.294
2.288
2.281
Pd–Cl
2.315
2.301
0.021
0.019
0.000
−0.001
0.010
0.000
0.021
ΔPd–Cl
0.000
0.000
8
9
12-PdCl2
22-PdCl2
Pd–NL
2.018
2.015
Pd–NL
2.018
2.015
Pd–O
2.003
2.009
Pd–O
2.003
2.009
ΔPd–O
0.000
0.000
10
11
12-Pd(OAc)2
22-Pd(OAc)2
In general, the recorded catalytic outcomes represent new perfor-
mance heights for N-donor-supported precatalysts and hold potentials
for industrial applications because of friendly temperature and time
conditions (c.f. Table 1, see Supplementary information S6 for NMR
spectral stacks of catalysis reaction mixtures under varying tempera-
tures).
a
NL = azole N-donor, NS = acetonitrile or benzonitrile N-donor. Structures
of 3-PdCl2MeCN, 7-PdCl2MeCN and 9-PdCl2MeCN are displayed in the
Supplementary information S3.
Consequently, it could be concluded for the in situ experiments, that
the better catalyst performances is attributable to factors that prevent
tendency for bis-ligand palladium species (i.e. lower donor strengths
and higher ligand bulkiness). On the other hand, stronger donor
strengths and smaller ligand sizes as for ligands 1 and 2 can be blamed
for the higher tendency for in situ generation of trans-bis-ligand co-
ordination species similar to those in Table 1 above.
Coupling efficiencies of the synthesized palladium precatalysts
Mono-ligand versus trans-bis-ligand precatalysts: Each of the
palladium complexes were tested as precatalysts (0.2 mol % loading) in
the coupling of (4-bromophenyl)methanol (1 mmol) and (4-acet-
ylphenyl)boronic acid (1.2 mmol). The precatalysts were compared at
45 °C and 15 min stirring durations in water / ethanol mixture (4 ml of
1:3) and in the presence of K2CO3 (1.5 mmol). The results are sum-
marized in Table 6.
Determining catalysis parameters for study of the complex precatalysts
Firstly, reaction parameters like time, temperature, additive base
type and base loading were varied. Under 45 °C and using substrates
and K2CO3 amounts as in the above in situ experiments, but with
0.2 mol % of complex 6-PdCl2MeCN, the yields increase from 42%
within 2 min stirring to 85% after 60 min, which indicates living cata-
lyst behaviour (Fig. 5). The duration of 15 min was chosen for further
First and foremost, a very sharp contrast is seen between the high
activities of the mono-ligand complexes 1-PdCl2MeCN – 9-PdCl2MeCN
(entries 1–11) and the inactivity of the trans-bis-ligand complexes 12-
PdCl2 – 22-Pd(OAc)2 (entries 12–18). This difference confirms that, for
a palladium complex to be catalytically useful at low temperature re-
gimes, coordinative saturation must be avoided. Furthermore, it proves
10

A.O. Eseola, et al.
MolecularCatalysis473(2019)110398
Fig. 4. Stacked absorption (ligand 4) and emission (ligand 2, 7 and 8) spectra of protonation-deprotonation equilibria. Insets show sigmoid fittings for plots of
absorption or emission intensities against pH (R2 > 0.99 in all cases).
to the desirable mono-ligand species. Based on literature observations
(Table 1), the bis-ligand complexes only become active at elevated
temperatures, which makes them less practical and industrially un-
attractive.
Table 3
Ligands pKa values.
Ligands and corresponding pKa values
Comparison among the mono-ligand precatalysts: It could also
be concluded from catalyses outcomes among the mono-ligand pre-
catalyst group (entries 1–11) that catalyst efficiencies are appreciably
better for the preformed mono-ligand complexes than for the in situ
‘PdCl2(MeCN)2 + ligand’ approach under identical reaction settings.
However, there is an observable interplay between ligand donor
strengths and substituent bulkiness, which is apparently in opposite
trends compared to the in situ alternative. In general, ligand donor
strength appears to show some dominance, but instances of steric
control are also observed.
8.47
3.60
0.23a
7.25
0.24a
5.10
0.04
0.07
N obtainableb
3.00
1.36
0.04
For instance, it is observed that 1-PdCl2MeCN (92%) yielded more
activity than 2-PdCl2MeCN (68%), which is ascribed to dominance of
donor strength among these 4,5-diethyl-1H-imidazole complexes.
Furthermore, gentle yield increase in correlation with increasing ligand
donor strengths is also identifiable for instance in comparison between
the complexes of the weaker donor oxazole ligands (8 and 9) and their
corresponding imidazole counterparts (6 and 7), which differ only by
ligand donor strength; i.e. 6-PdCl2MeCN (74%) > 8-PdCl2MeCN (56%)
as well as 7-PdCl2MeCN (81%) > 9-PdCl2MeCN (73%). However, in
some instances, complexes of the bulkier anthracene-substituted li-
gands generally performed better than their corresponding mesityl-
substituted analogues; e.g. 7-PdCl2MeCN (81%) > 6-PdCl2MeCN (74%)
and 9-PdCl2MeCN (73%) > 8-PdCl2MeCN (56%).
2.32
a
0.01a
0.90
0.05a
0.07a
In this case, fluorescence spectra were used to estimate pKa values.
bComplicated spectral changes suggest simultaneous protonation at multiple
points.
that complexes of monodentate N-donor ligands in mono-ligand co-
ordination environment as designed in this work are capable of pro-
viding inexpensive and yet efficient catalyst systems, which can over-
come problems of costs and other problems associated with phosphine-
and organometallic-based catalyst systems.
Finally, substitution of acetonitrile by benzonitrile as co-ligand in
the precatalysts produced adverse catalytic outcome despite their si-
milar basicity values; i.e. entries 6–9, 6-PdCl2MeCN (74%) > 6-
PdCl2PhCN (53%) and 7-PdCl2MeCN (81%) > 7-PdCl2PhCN (43%).
It is also noteworthy that the only appreciable yield recorded from
the bis-ligand group resulted from 32-PdCl2, which is attributable to the
benefit of ligand-ligand steric repulsion that aids ease of decomposition
11

A.O. Eseola, et al.
MolecularCatalysis473(2019)110398
Table 4
Catalytic yields for ‘ligand + PdCl2(MeCN)2’ in situ catalyst generation at 45 °Ca.
Ligand
Time (min)
Yield (%)
TOF (h−1
)
1
15
15
15
15
15
2
18
8
360
2
160
3
39
58
50
35
47
59
67
78
40
62
35
17
35
780
4
1160
1000
700
5
6
6
4
940
6
8
1180
1340
1560
800
6
15
20
15
15
15
15
15
6
7
8
1240
700
9
PPh3
None
340
700
a
Reaction conditions: (4-Bromophenyl)methanol (1 mmol), (4-acetylphenyl)boronic acid (1.2 mmol), K2CO3 (1.5 mmol), PdCl2(MeCN)2 (0.2 mol %), ligands
(0.4 mol %), in water / ethanol mixture (4 ml of 1:3), 45 °C, 15 min. Yields were determined by 1H-NMR.
Table 6
Catalyst performance of the synthesized palladium complexes grouped as
mono-ligand, bis-ligand dichloride and bis-ligand diacetate complexesa.
Entry
Precatalyst
Yield (%)
TOF(h−1
)
1
1-PdCl2MeCN
2-PdCl2MeCN
3-PdCl2MeCN
4-PdCl2MeCNcis
5-PdCl2MeCN
6-PdCl2MeCN
6-PdCl2PhCN
7-PdCl2MeCN
7-PdCl2PhCN
8-PdCl2MeCN
9-PdCl2MeCN
92
68
69
80
69
74
53
81
43
56
73
1840
1360
1380
1600
1380
1480
1060
1620
860
2
3
4
5
6
7
8
9
10
1120
11
1460
12
13
14
15
16
12-PdCl2
22-PdCl2
32-PdCl2
62-PdCl2
PdI2(PPh3)2
18
360
Trace
61
Trace
1220
180
Fig. 5. Plot of yield against reaction time for 6-PdCl2MeCN as catalyst pre-
9
cursor.
Nill
Nill
17
12-Pd(OAc)2
Nill
Nill
Table 5
18
22-Pd(OAc)2
3
60
Catalytic performance under varying reaction conditions using precatalyst 6-
PdCl2MeCNa.
Time(min)
a
Reaction conditions: Precatalyst (0.2 mol %), (4-bromophenyl)methanol
(1 mmol), (4-acetylphenyl)boronic acid (1.2 mmol), 45 °C, 15 min stirring,
K2CO3 (1.5 mmol), water / ethanol mixture (4 ml of 1:3). Yields were de-
termined by 1H-NMR.
Temp. (oC)
Base
Yield (%)
TOF (h−1
)
15
15
15
15
15
15
20
45
55
K2CO3
K2CO3
K2CO3
35
74
84
700
1480
1680
conditions of 45 °C and 15 min stirring as above, the scope of catalytic
performance with other substrates was tested and the results in Scheme
5 reveal high yields with substrate tolerance. It could therefore be
concluded that the following features are beneficial for achieving am-
bient condition catalytic performance by the monodentate N-donors: (i)
higher ligand donor strengths, (ii) careful implementation of steric
features that discourages trans-bis-ligand complexation and (iii) use of
smaller sized monodentate co-ligands of weaker donor strengths.
70
45
45
K2CO3
No base
K3PO4
98
1960
Nill
73
Nill
1460
1
45 (MW)
K2CO3
54
16200
15
45 (H2O)
K2CO3
89
1780
a
Reaction conditions: (4-Bromophenyl)methanol (1 mmol), (4-acetylphenyl)
boronic acid (1.2 mmol), base (1.5 mmol), 6-PdCl2MeCN (0.2 mol %). Yields
were determined by 1H-NMR.
Results for Suzuki coupling for aryl chlorides and Heck coupling
Thus, it could be considered that the larger size of the benzonitrile co-
ligand can be considered as tending towards behaviour of the trans-bis-
ligand complexes (entries 12–18).
Selected complexes from the mono-ligand group were subjected to
test for coupling aryl chlorides and results are presented in Table 7. It is
interesting to observe that, in the coupling of the activated 1-chloro-4-
Substrate scope: Using 6-PdCl2MeCN under same ambient
12

A.O. Eseola, et al.
MolecularCatalysis473(2019)110398
Scheme 5. Coupling of various substrate combinations using 6-PdCl2MeCN as catalyst source (phenyl ring from aryl bromides appear on the left while the ring from
arylboronic acids appear on the right; yields were determined by 1H-NMR).
and their N-donor strengths, which were spectroscopically determined
as pKas, were systematically varied from 0.9 to 8.5 via substituent
Table 7
Catalyst efficiencies for aryl chloride experiments by selected mono-ligand
complexesa.
variations. Target mono-ligand complexes (1-PdCl2MeCN
– 9-
PdCl2MeCN, 6-PdCl2PhCN and 7-PdCl2PhCN) as well as trans-bis-ligand
complexes (12-PdCl2, 22-PdCl2, 32-PdCl2, 62-PdCl2, 12-Pd(OAc)2 and
22-Pd(OAc)2) were isolated and used along with PdI2(PPh3)2 to study
catalysis trends. Surprise oxidative decomposition and 1,2-phenyl shift
on 4,5-imidazole position of a ligand caused by the commercialized
precatalyst Pd2(dba)3 was structurally confirmed and presented herein.
With coupling efficiency reaching up to 100% (TOF ≈ 2000) at 45 °C
within 15 min coupled with substrate functional group tolerance, it could
be concluded that the mono-ligand precatalyst designs supported by li-
gands 1–9 represent a true catalyst improvement initiative, which
structurally and energetically places the complexes at catalytic ad-
vantage above the coordinatively saturated analogues! Trans-bis-ligand
coordination utterly hinders ambient temperature catalyst efficiencies.
Even the commercialized PdI2(PPh3)2 displayed no yield under the stu-
died reaction setting. Consequently, presence of multiple ligands on a
palladium centre is responsible for the undesirable and avoidable need of
high temperatures, long reflux durations and unnecessarily high catalyst
loadings. Thus, implementing two or more ligand equivalents to palla-
dium either via in situ or as precatalysts, should be strongly discouraged.
Furthermore, the sterically bulky or rigid substituents around the
ligand N-donor atom mainly favours the catalysis by employing ligand-
ligand repulsion to hinder bis-ligand coordination. Correlation between
catalyst activities and ligand donor strengths revealed somewhat op-
posite trends between in situ approach and the preformed complexes.
While in situ catalyst generation worked better with weaker donor and
bulkier ligands (i.e. implying reluctance for bis-ligand complexation),
mono-ligand complex precatalysts isolated from stronger donors pos-
sessed superior activities. Furthermore, these mono-ligand precatalysts
also coupled activated aryl chloride from 60 °C while Heck coupling
activities were observed only at the early minutes of reactions. In
general, the results serve as motivation to further hunt for monodentate
N-donor superbase ligands designed with appropriate steric demands.
Precatalyst
Temp. (oC)
Time
Yield
1-PdCl2MeCN
2-PdCl2MeCN
2-PdCl2MeCN
2-PdCl2MeCN
2-PdCl2MeCN
2-PdCl2MeCN
4-PdCl2MeCN
6-PdCl2MeCN
7-PdCl2MeCN
100
45
30
30
30
8 h
30
30
30
30
30
71
Trace
22
60
60
47
80
62
100
100
100
100
78
46
44
47
a
Reaction conditions: Precatalyst (0.2 mol %), 1-chloro-4-nitrobenzene
(1 mmol), (4-acetylphenyl)boronic acid (1.2 mmol), K2CO3 (1.5 mmol), water /
ethanol mixture (4 ml of 1:3). Yields were determined by 1H-NMR.
nitrobenzene, appreciable yields could be achieved with living catalyst
behaviour from 60 °C in the presence of 2-PdCl2MeCN (22% yield in
30 min and 47% yield in 8 h). At 80 °C and 100 °C, 62% and 78% yields
were respectively obtained within 30 min. See Supplementary in-
formation S7 for NMR spectral stack showing product increase from 40
to 100 °C for 30 min’ reactions. Comparing the performance by various
precatalysts, the importance of ligand donor strength is again empha-
sized by trends of catalysis yields. Under 30 min at 100 °C, 1-
PdCl2MeCN (71%) and 2-PdCl2MeCN (78%) with stronger N-donor li-
gands performed notably better compared to 4-PdCl2MeCN (46%), 6-
PdCl2MeCN (44%) and 7-PdCl2MeCN (47%).
Efforts made to test Heck coupling potentials of the synthesized
complexes revealed that the complexes could couple terminal olefins
and aryl bromides, but these precatalysts portray a doubtful living
catalyst character since most of the conducted catalytic runs yielded
between 20 and 30% products with very little yield increase as time
increased. The corresponding catalysis data is available in the
Supplementary information Table S3.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Conclusion
A.O.E is grateful to the Alexander von Humboldt Foundation for
support through Georg-Forster postdoctoral scholarship and to
Redeemer’s University for fellowship leave. The financial support by
the Deutsche Forschungsgemeinschaft (DFG) is gratefully acknowl-
edged (PL 155/11, PL 155/12 and PL155/13).
In a bid to develop new palladium catalyst materials with structural
proximity to their active state compositions, a series of monodentate
imidazole (1 – 7) and oxazole (8 – 9) N-donor ligands were synthesized
13

A.O. Eseola, et al.
MolecularCatalysis473(2019)110398
Appendix A. Supplementary data
coupling of aryl esters at room temperature using a practical palladium-NHC
(NHC]N-heterocyclic carbene) precatalyst, Adv. Synth. Catal. 360 (2018)
1538–1543.
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2019.110398.
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